Horizontal Transfer of Nodulation Genes in soils and Microcosms from Bradyrhizobium japonicum to B. elkanii

We investigated the horizontal transfer of nodulation (nod) genes to a Bradyrhizobium elkanii strain, lacking common nod genes as a recipient, in soils and microcosms using selection systems of antibiotic resistance and legume nodulation. We observed the horizontal transfer of nod genes at 4°C in Nakazawa soil where peculiar strains (HRS strains) of B. japonicum harboring high copy numbers of insertion sequences dominated. In microcosms containing HRS strains as donors, we detected a similar horizontal transfer from B. japonicum HRS strain NK5 to the B. elkanii recipient more efficiently at 4°C, which was verified by examining hybridization, nodulation and Nod factor production. These traits were, however, gradually lost during successive cultures. Plasmid analysis indicated that this event was not due to the simple transfer of plasmid carrying common nod genes. These results suggest the potential for horizontal transfer of nod genes among bradyrhizobia and other bacterial populations in soil environments.